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Author: Subject: I need a vacuum bell. And I'm cheap.
jgourlay
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[*] posted on 12-10-2009 at 08:25
I need a vacuum bell. And I'm cheap.


Gents, do any of you all have a an idea for a cheap(ish) vacuum bell? I have a vacuum pump and was fine with paying $200 for that.

But I'm just having a hard time swallowing $105+ for a thick version of cake cover. Before I spend $500 and a month of weekends on various unsuccessful attempts to homejob something, I thought I'd see what other had come up with.
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watson.fawkes
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[*] posted on 12-10-2009 at 09:03


Quote: Originally posted by jgourlay  
Gents, do any of you all have a an idea for a cheap(ish) vacuum bell?
Does it need to be transparent?
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1281371269
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[*] posted on 12-10-2009 at 09:53


How about putting the bell in the bottom of a 1l RBF and attaching the pump to that. It wouldn't really look the part but it would work and you would be using only equipment you already had.
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bquirky
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[*] posted on 12-10-2009 at 21:52


How about a round salad bowl.. they are fairly thick and have a roundish shape that might be able to take some pressure..

I pulled a bit of a vacuum with a small two stage displacement pump using a jam jar. I doubt the vacuum was that great but it was enough to light up some nice plasma inside. and i didn't have any trouble with the jar fracturing but it was fairly small perhaps 400ml the
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Contrabasso
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[*] posted on 13-10-2009 at 10:28


Depends what volume and quality of vacuum you want to contain (!).
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Mr. Wizard
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[*] posted on 13-10-2009 at 11:16


I will second bquirky's idea. Use some grinding compound or abrasives against a smooth steel surface to smooth out the mating surface. A minimalist method would be to get a sheet of glass twice as wide as the bowl mouth, use some fine abrasive and water while sliding the bowl rim around on the glass. Be sure to rotate the bowl occasionally while working. You will be able to see the surface smooth out, as the higher points are worn down. An even more minimalist method would be to use a smooth piece of a poured concrete sidewalk or floor. Take a minute to find the smoothest part with a straight edge.

The base for the bowl vacuum chamber can NOT be a thin sheet of glass, it will break. A thick piece of steel or aluminum would work well. Here is a link on how to smooth out a piece of steel. The method mentioned in the comments, using abrasives, may be of interest.

http://gfish.livejournal.com/187606.html
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merrlin
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[*] posted on 13-10-2009 at 12:47


If you are going to use a glass object with an unknown composition and unknown thermal history, I recommend using a wire mesh cage to protect against implosion shrapnel, particularly if the bell jar is going to be subjected to heat. The thermal expansion of glasses can vary widely, as can the residual stresses after fabrication. If you introduce stress concentrations by grinding a surface that has a residual tensile stress, you will be inviting failure unless you anneal it after grinding.
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Mr. Wizard
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[*] posted on 13-10-2009 at 13:05


Yes, a wire cage and maybe even some cloth , (towels, old sheets, anything cloth) over the cage would be a great and cheap safety precaution for any glass bell jar, especially a home made one.
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thefips
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[*] posted on 17-10-2009 at 07:18


just take the glas "window" from an old washing machine with a front door.
It has a broad edge, where you can seal it with silicone. The glas is quite thick, but for save operation put a plate of pmma between your vacuum apparatus and you.
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franklyn
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[*] posted on 19-10-2009 at 00:09


If a glass gallon wine jug will do , then just cut off the base
by scoring first then heating and rapid quenching , it should
drop right off.

.
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